Astro 100 Chapter 2.pptx - Astronomy 100 Jaquelin Erazo...

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Astronomy 100Jaquelin ErazoCollege of Staten Island
Chapter 2 – Observing the Sky: The Birth ofAstronomy
2.1 - The Sky AboveWe feel like the Earth is at the center of theuniverse (geocentric view)Believed until the European RenaissanceReinforced certain philosophical and religioussystemsBUT THIS IS INCORRECT – why?
The Sky Around Us
Early Greeks regarded the sky as acelestialsphereObjects are fixed and rotates around Earth on anaxis (incorrect)As the celestial sphere rotates, the objects withinit maintain their positions2.1 - The Sky Above
Earth axis: an imaginary line that connects the Northand South PolesEarth rotates around its axisImagine an extension of Earth’s axis and intersectthe celestial spherePoints of intersection:north celestial poleandsouthcelestial poleThe sky appears to turn in the opposite direction aroundthecelestial polesCelestial equatoris like the Earth’s equatorlies between the celestial poles2.1 - The Sky Above
Celestial Sphere
Apparent motion of the celestial sphere is dependenton latitude (north or south of equator)North/South pole: circle around celestial pole, none rising orsettingOnly half of the sky is visible to the observer (north-north, south-south)Equator: all stars rise and set from east to westIn between: the N celestial pole is at some position betweenoverhead and the horizon. Its angle above the horizon turnsout to be equal to the observer’s latitude. Stars rise and setat an angle to the horizon.E.g. 38⁰ N, stars within 38⁰ of the North pole can never set (alwaysabove the horizon)2.1 - The Sky Above
Celestial Sphere Movement
Rotating Sky LabLink:s/ce_hc.html
Stars continue to circle during the day, but thesun makes it difficult to seeWe can think of the Sun being located at aposition on the hypothetical celestial sphereSun rises and sets (rises above horizon and fallsdown)Changes position gradually, moving each day 1⁰east relative to the stars -> Sun is moving aroundthe Earth, taking 1 year to go around2.1 - The Sky Above
Ecliptic:the apparent annual path of the Sun on thecelestial sphereTime passes, we look at the Sun from differentplaces in our orbit, we see it projected againstdifferent places in our orbit -> different stars in thebackground (if we could see stars in the daytime)After a year: Earth has completed one trip aroundthe Sun, the Sun will appear to have completed onecircuit of the sky along the ecliptic.2.1 - The Sky Above
Constellations & the Sun
The Celestial Tilt
Fixed stars: stars that maintain fixed patterns throughmany generationsWandering stars: independent motion compared tofixed stars; also called aplanet“planet” in ancient Greek means “wanderer”Moon, Sun, planetsThe word “planet” used here is not the same as the termwe use it in modern astronomyZodiac: planets, Sun, and Moon found in the skywithin a narrow 18-degree-wide belt, centered on theecliptic2.1 - The Sky Above

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Term
Fall
Professor
Robbins
Tags
Planet

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